The present invention relates to laser printers and more particularly to a color laser printer in which toner images of different colors are superposed on an intermediate transfer member to provide a color image.
In a conventional color laser printer of the type in which toner images of respective colors are superposed on an intermediate transfer member so as to be recorded thereon, the accuracy of register of toner images of respective colors to be superposed on the intermediate transfer member must be improved in order to obtain a color image of high quality. In JP-A-62-195687, in an effort to improve the register accuracy, drive means for driving a photosensitive member and means for driving an intermediate transfer member are controlled in synchronism with a common clock signal. Further, the timings for exposure and transfer of respective colors and printing synchronization timing signals corresponding to a printing area (a printing permission signal in the main scanning direction vertical to the rotation direction of the photosensitive member, that is Hsync, and a printing permission signal in the auxiliary scanning direction which coincides with the rotation direction of the photosensitive member, that is, Vsync) are controlled using the clock signal. The printing area is the same for the respective colors and the printing synchronization timing signal for one color is controllably generated every revolution of the intermediate transfer member in synchronism with the rotation of the intermediate transfer member.
When the photosensitive member has the form of a belt, a belt to be used is preferably endless but from the standpoint of production cost, a seamed belt is used more frequently than an endless belt. With the seamed belt used, an electrostatic latent image cannot be formed on a seam and hence the position of the seam is always managed to control the formation of an electrostatic latent image such that the electrostatic latent image is formed at a position other than the seam.
In the above prior art, the process for transfer of toner images onto the intermediate transfer member is effected sequentially plural times to form a color image and the toner images are transferred collectively to a paper sheet, thus making a printing area dependent on the outer peripheral length of the intermediate transfer member. But when only a toner image of single color is printed, the superposition of toner images on the intermediate transfer member is not effected and as soon as the toner image formed on the photosensitive member is transferred to the intermediate transfer member, it is transferred to a paper sheet and therefore the printing area does not depend on the outer peripheral length of the intermediate transfer member.
When toner images of a plurality of colors are superposed on the intermediate transfer member to form and print a color image, the toner images are superposed on the intermediate transfer member and therefore with a toner image formed at an area which is longer than the outer peripheral length of the intermediate transfer member, toner images of that color are superposed. This implies that the maximum length of the printing area is limited to the outer peripheral length of the intermediate transfer member.
More particularly, when printing a color image formed by superposing toner images of a plurality of colors to a paper sheet of a size which is shorter than the outer peripheral length of the intermediate transfer member, printing to a desired area in the direction of the paper sheet length is allowed. However, when printing a color image formed by superposing toner images of a plurality of colors to a paper sheet of a size which is longer than the outer peripheral length of the intermediate transfer member, there arises a problem that the maximum size of a printing permissible area in the direction of the paper sheet length is limited approximately to the outer peripheral length of the intermediate transfer member.